Again, IBM terminology is nonstandard. Here is how the textbook describes
the general purpose registers, which are used to store binary data.
“Each register contains 32 data bits, numbered from left to right as 0 through 31.
… For binary values, bit 0 (to the left) is the sign bit and bits 1 – 31 are data.”
The IBM documentation will tend to discuss 32–bit signed integers as
“31 bit data”. This makes sense; it is just not the present–day standard.